1. Technical Field
The invention relates to a process for recovering hydrocarbons from a subterranean formation, and more particularly to a hydrocarbon recovery process using a permeability-reducing agent.
2. Background Information
The utility of crosslinked polymer gels as a permeability-reducing agent in subterranean hydrocarbon-bearing formations to facilitate hydrocarbon recovery therefrom has long been known in the art. U.S. Pat. No. 3,762,476 to Gall is representative of conventional teaching in the art of its time. Gall discloses a conformance improvement treatment process, wherein a permeability-reducing crosslinked polymer gel is formed in situ by injecting an aqueous slug containing a crosslinkable water-soluble polymer into a subterranean treatment region followed in sequence by an aqueous slug containing a crosslinking agent made up of a polyvalent metal cation complexed with a retarding anion. Useful polyvalent metal cations listed in Gall include iron(II), iron(III), aluminum(III), chromium(III), calcium(II), magnesium(II). Useful retarding anions complexed with the cation include citrate, phosphate. acetate, nitrilotriacetate, and tartrate.
Sequential injection of the gel components as separate and distinct slugs into the treatment region is essential to the teaching of treatment processes such as Gall insofar as surface contacting of the polymer and crosslinking agent was believed to cause premature gelation of the gel components prior to reaching the treatment region. If prematurely formed at the surface or in the well bore, placement in the treatment region of the gels taught by Gall was difficult, if not impossible, to achieve. Consequently, sequential injection of the gel components in concept avoided premature gelation by delaying contacting of the components until they were displaced out into the treatment region.
In situ contacting of the polymer and crosslinking agent as required by Gall, nevertheless, proved operationally unattractive in many hydrocarbon recovery applications because of the difficulty in achieving adequate mixing of the gel components in situ. Without adequate mixing, the gels of Gall were poorly formed, resulting in weak and unstable gels that performed ineffectively as permeability-reducing agents.
In the face of conventional teaching represented by Gall, U.S. Pat. No. 4,683,949 to Sydansk et al. identified specific gel components and gelation parameters for a crosslinked polymer gel having utility in a conformance improvement treatment process, whereby the polymer and crosslinking agent of the gel could be mixed at the surface in a homogeneous gelation solution and placed in the treatment region by injection therein as a single slug. Thus, Sydansk et al. overcame the inherent operational limitations of processes such as Gall that required sequential injection and in situ mixing of the gel components. Sydansk et al. was predicated on the finding that a chromium(III)/carboxylate complex crosslinking agent could be mixed with a crosslinkable polymer at the surface to form a gelation solution producing gels that were uniquely stable and had highly predictable gel strengths and gelation times, yet were capable of practical long-term placement in subterranean treatment regions, even in hostile environments. The process of Sydansk et al. successfully enabled formulation of gels having a broad range of physical characteristics for effective conformance improvement treatments under varying in situ conditions.
The teaching of Sydansk et al. has subsequently been adapted to certain specific, typically problematic, conformance improvement treatment applications. For example, in some high temperature or far well bore applications, it has been found that absent the addition of a supplemental delaying agent to the gelation solution, the chromium(III)/carboxylate complex taught by Sydansk et al. excessively crosslinks the polymer before the gelation solution reaches the desired treatment region, increasing the difficulty of placing the gel in the treatment region.
Accordingly, U.S. Pat. No. 4,706,754 to Smith addressed specific treatment applications, wherein longer gel times are advantageously provided. Smith discloses the addition of a supplemental delaying agent to a gelation solution containing a water-soluble crosslinkable polymer and a metal/carboxylate complex crosslinking agent. The supplemental delaying agent disclosed by Smith comprises a plurality of carboxylate anions in the form of a carboxylic acid that delays gelation of the polymer and crosslinking agent, thereby facilitating heretofore problematic placement of the crosslinked gels in high temperature or remote treatment regions.
U.S. Pat. Nos. 5,131,469 and 5,143,958 to Lockhart et al. likewise teach gelation solutions containing a crosslinkable polymer, a crosslinking agent, and a plurality of carboxylate anions in the form of a carboxylic acid that performs as a delaying agent. The Lockhart et al. patents, however, teach a crosslinking agent having a crosslinking metal cation in the form of a water-soluble inorganic chromium(III) salt, as distinguishable from Smith wherein the metal cation of the crosslinking agent is in the form of an organic metal/carboxylate complex. The Lockhart et al. patents are further distinguishable from Smith insofar as the Lockhart et al. patents expressly repudiate the utility of the acetate anion as a component of the gelation solutions taught therein, whether the acetate anion is in the form of a chromium(III) complex, acetic acid, or an acetate salt.
Notwithstanding the teaching of the prior art represented by the U.S. patents of Smith and Lockhart et al., a relatively less acidic gelation solution has been found with a controllable gel time to enable placement of the resulting gel in a wide range of subterranean treatment regions. Moreover, the present gelation solution obviates problems experienced by the prior art, wherein acidic gelation solutions are often corrosive when contacting surface or well bore equipment and tubing. The present gelation solution obviates another problem encountered with acidic gelation solutions, wherein certain polymers, otherwise useful in gelation solutions, tend to autohydrolyze in an acidic environment. Acid catalyzes the autohydrolysis reaction of the polymer, at times causing premature gelation of the gelation solution. Acidic gelation solutions can also react with the reservior rock, particularly carbonates, acidizing the rock which causes loss of the acid component and which also causes complicated and undesirable side reactions with the rock. As such a need has existed for an effective gelation solution that is relatively noncorrosive to the oil field tubing and equipment, does not promote autohydrolysis of the polymer to be crosslinked, and is substantially unreactive with the reservior rock, yet can be formulated with a sufficient gel time for placement in remote or high temperature treatment regions.
It is, therefore, an object of the present invention to provide a process forming a crosslinked polymer gel for permeability reduction in a treatment region. It is also an object of the present invention to provide a gelation solution composition that forms such a crosslinked polymer gel. It is a further object of the present invention to retard the gelation rate of the gelation solution by supplementing the gelation solution with a delaying agent. It is another object of the present invention to control the degree of gelation rate retardation in a predictable and controllable manner by selectively increasing or decreasing the concentration of delaying agent in the gelation solution and correspondingly adjusting the pH of the gelation solution. It is another object of the present invention to control the degree of gelation rate retardation by selecting the ligand strength of the delaying agent relative to the ligand strength of the crosslinking agent. It is still another object of the present invention to provide such a gelation solution that is relatively noncorrosive to the oil field tubing and equipment. It is yet another object of the present invention to provide such a gelation solution that does not tend to induce autohydrolysis of the crosslinkable polymer contained therein. It is another object of the present invention to provide such a gelation solution that is substantially unreactive with the reservior rock.